Semiconductor devices are typically fabricated on a substrate that provides mechanical support for the device and often contributes to the electrical performance of the device as well. Silicon, germanium, gallium arsenide, sapphire and silicon carbide are some of the materials commonly used as substrates for semiconductor devices. Many other materials, including semiconductor as well as non-semiconductor materials, may also be used as substrates for semiconductor devices. Semiconductor device manufacturing typically involves fabrication of many semiconductor devices on a single substrate.
Substrates are typically formed in the shape of circular wafers having a diameter presently ranging, for example, from less than 1 inch (2.54 cm) to over 12 inches (30.5 cm) depending on the type of material involved. Other shapes such as, for example, square, rectangular or triangular wafers are possible, however. In addition, broken and/or irregular wafers may still be used to fabricate semiconductor devices. Semiconductor devices are formed on the wafers by the precise formation of thin layers of semiconductor, insulator and metal materials that are deposited and patterned to form useful semiconductor devices such as diodes, transistors, solar cells and other devices.
Individual semiconductor devices may be relatively small compared to the size of the wafer on which they are formed. For example, a typical light emitting diode (LED) chip such as the C430-XB290 LED chip manufactured by Cree, Inc., in Durham, N.C. measures only about 290 microns by 290 microns square (1 micron=0.0001 cm). Accordingly, a very large number of LED chips (also referred to as “dice”) may be formed on a single 2 inch (5.08 cm) diameter silicon carbide (SiC) wafer. After the dice are formed on the wafer, at least some of the individual dice are generally separated so that they can be mounted and encapsulated to form individual devices. The process of separating the individual dice is sometimes referred to as “dicing” the wafer or “singulating” the dice.
Dicing a wafer into individual semiconductor devices may be accomplished by a number of methods. One method of dicing a wafer involves mounting the wafer on an adhesive surface and sawing the wafer with a circular saw. A series of closely spaced saw cuts is made first in one direction and then in a second direction perpendicular or oblique to the first direction. Thereby, a number of individually diced, square, rectangular, or other regularly shaped devices are produced. Other methods of dicing, such as “scribe-and-break” are possible. However, sawing may be preferable for certain applications and substrate types. In particular, for the fabrication of LEDs on silicon carbide substrates, sawing or partially sawing the substrate may be preferable.
Sawing is typically performed using an abrasive circular saw blade under a flow of purified water. The water may assist with the removal of both heat and unwanted byproducts of the sawing process, such as silicon carbide dust.
It is generally desirable to avoid or reduce contamination of the semiconductor wafer. As is well known in the semiconductor art, contamination of a semiconductor wafer may have a number of detrimental effects on semiconductor devices formed from the wafer. For example, contamination may reduce device performance or even render a device non-functional. In the case of LEDs, contamination may cause a device to have a lower light output and/or a higher operating voltage. Contamination may also harm the metal contacts on an LED device, potentially making it more difficult to package the device reliably and/or reducing light output from the device.
During the sawing operation, contaminants may be introduced to the semiconductor wafer from a number of sources such as, for example, the mounting tape, the saw, the water, and even from the substrate itself in the form of sawdust. Contamination is typically addressed by increasing the purity of the materials used in the sawing process, for example, by using higher purity water and/or tape that contains fewer contaminants, and/or by cleaning the wafers prior to sawing. Protective layers have also been used to reduce contamination. However, even when using such measures, it may be difficult to reduce contamination to a desirable level in conventional sawing systems. Accordingly, it may be desirable to reduce contamination to the substrate and associated semiconductor devices that may result from a sawing operation.